At least some known gas turbine engines combust a fuel air mixture to release heat energy from the mixture to form a high temperature combustion gas stream that is channeled to a turbine via a hot gas path. The turbine converts thermal energy from the combustion gas stream to mechanical energy that rotates a turbine shaft. The output of the turbine may be used to power a machine, for example, an electric generator, pump, or the like.
At least one by-product of the combustion reaction may be subject to regulatory limitations. For example, within thermally driven reactions, nitrogen oxide (NOx) may be formed by a reaction between nitrogen and oxygen in the air initiated by the high temperatures within the gas turbine engine. Generally, engine efficiency increases as the combustion gas stream temperature entering a turbine section of the gas engine increases; however, increasing the combustion gas temperature may facilitate an increased formation of undesirable NOx.
Combustion normally occurs at or near an upstream region of a combustor that is normally referred to as the reaction zone or the primary zone. Inert diluents may be introduced to dilute the fuel and air mixture to reduce peak temperatures and hence Nox emissions. However, inert diluents are not always available, may adversely affect an engine heat rate, and may increase capital and operating costs. Steam may be introduced as a diluent but may also shorten the life expectancy of the hot gas path components.
In an effort to control NOx emissions during turbine engine operation, at least some known gas turbine engines use combustors that operate with a lean fuel/air ratio and/or with fuel premixed with air prior to being admitted into the combustor's reaction zone. Premixing may facilitate reducing combustion temperatures and hence NOx formation without requiring diluent addition. However, if the fuel used is a process gas or a synthetic gas, there may be sufficient hydrogen present such that an associated high flame speed may facilitate autoignition, flashback, and/or flame holding within a mixing apparatus. Premix nozzles also have reduced turndown margin since very lean flames can blow out.
To extend turndown capability, premix nozzles are employed which utilize a diffusion tip to inject fuel for start-up and part-load conditions. A diffusion tip is typically attached to the center body of the premix nozzle. Syngas combustors also use stand-alone diffusion nozzles to burn a variety of different fuels to prevent flame holding/flashback with high hydrogen fuels and blow out with low Wobbe index fuels. A shortcoming in these systems is high NOx levels when running in pilot or piloted premix mode. Currently, co-flow diffusion tips are utilized to provide pilot flames for stability, turn down capability and fuel flexibility. This arrangement, however, also results in high NOx.
A lean direct injection (LDI) method of combustion is typically defined as an injection scheme that injects fuel and air into a combustion chamber of a combustor with no premixing of the air and fuel prior to injection similar to traditional diffusion nozzles. However, this method can provide improved rapid mixing in the combustion zone resulting in lower peak flame temperatures than found in traditional non-premixed, or diffusion, methods of combustion and hence, lower NOx emissions